EUV: Unlike anything else in the fab

By Pete Singer, Editor-in-Chief

Imagine EUV lithography in high volume production. ASML has been working for years to make it happen.

Earlier this year, ASML said that one of its major chip-manufacturing customers has placed an order for 15 EUV systems, including two that are set to be delivered before the end of this year. ASML did not name the customer, but it is almost certainly Intel (according to research firm IHS).

ASML’s CEO Peter Wennink said in a statement announcing that the customer agreement had been signed: “EUV is now approaching volume introduction. Long-term EUV planning and EUV ecosystem preparation is greatly supported by this commitment to EUV, kick-starting a new round of innovation in the semiconductor industry. The commitment extends the planning horizon and increases the confidence in EUV.”

Unlike the current atmospheric based High End immersion lithography tools used in volume manufacturing, the ASML NXE tool is vacuum based and using 13.5nm EUV light, generated by a tin-based laser produced plasma source. The systems feature all-reflective 4x reduction optics assemblies from Carl Zeiss SMT with a numerical aperture (NA) of 0.33 and a maximum exposure field of 26mm by 33mm.

EUV tools are very different from any other tool in a fab in a couple of different ways. A main difference is that the tool is designed to operate in a continuous mode. “Other tools in the fab, such as single wafer tools or batch tools, will undergo many step changes during a total cycle such as process, vent, load and unload wafers and also cleaning steps,” says Jos Donders, global market sector manager at Edwards. “In principle the EUV tool is made for continuous operation. Knowing the cost of the tool and the cost for the facilities, you understand why it’s so important that the tool is always up and why there is such a demand on the reliability and uptime of the supporting equipment such as vacuum and abatement.”

Donders, who was involved with the early work at ASML in understanding vacuum and abatement requirements of EUV, said the scanner and the source have very different requirements when it comes to vacuum levels. “The condition in the source is very different than the condition in the scanner. The challenge for the vacuum and abatement system is to handle the different conditions in an acceptable footprint in the sub-fab,” he said. “The cleanliness requirements, the materials selection and the overall budget are very important, as is the vacuum system that supports it,” he added.

Hydrogen in EUV is used to mitigate the contamination effect on the mirrors Andrew Chambers, Technical manager at Edwards said.

Pumping hydrogen is a challenge in itself. “It’s a small molecule,” says Donders. “It’s very difficult to pump. Your pumping mechanism needs to accommodate hydrogen, but also other gases (when the tool is in different states).” Chambers said there is interest in alternative solutions for handling and abating the process gases for EUV and work in Edwards is underway to achieve this ahead of volume manufacturing.

Donders concluded that one of Edwards’ main tasks is to enable EUV lithography going into volume production by supporting it needs to further improve the total energy use and offering sustainable solutions going forward.

Hydrogen is used for protecting the EUV multi-layered mirrors from being contaminated by tin (the ASML EUV source is a LPP tin-based source – tin enables an EUV wavelength of 13.5 nm). Helium is too expensive, considering the large volume of mitigation gas that is required in such a large machine. As for nitrogen, like air, it absorbs EUV radiation and thus cannot be used.

Hydrogen is used to remove tin from the mirror. It reacts with the tin to form volatile tin hydride (SnH4) vapor. When the laser beam hits the tin droplets, EUV radiation is produced but much of the tin is ablated and coats the mirror and much of the chamber as well.

Helium is not only expensive, but the molecules of Helium are very close they do not allow light to pass though them, for hydrogen the molecules are far apart light can pass easily between the molecules, the risk with hydrogen is that its highly flammable. Proper safety measures have to be taken with Hydrogen.

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